The present invention relates to an air-fuel ratio control system with an electrically-controlled carburetor in an intake system of an internal combustion engine and with a three-way catalytic converter in an exhaust system. The air-fuel ratio control system controls the air-fuel ratio of an air-fuel mixture to a stoichiometric air-fuel ratio at which the three-way converter operates most effectively and controls the air-fuel ratio to a fixed rich air-fuel ratio (small air-fuel ratio) at heavy load driving in the entire range of engine speed, so that the controlled air-fuel ratio range and the fixed air-fuel ratio range are adjusted properly to vehicle driving conditions.
A conventional air-fuel control system is provided with a detecting device for detecting heavy load operation of the engine and a control circuit for controlling the air-fuel ratio to a fixed rich ratio in order to produce high power for the heavy load operation.
In such a system, because the intake passage vacuum and throttle opening degree vary in dependency on the load, a certain value of the intake passage vacuum or throttle opening degree is selected for the detection of heavy load operation. For instance, in the load characteristic shown in FIG. 1, a curve A is the torque curve at full throttle and curve B shows one of torque-engine speed characteristics of a vacuum detecting device. Using the intake passage vacuum detecting method, in the range beneath the curve B, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio for partial power operation, but in the range above the curve B the ratio is fixed to a rich air-fuel ratio for high power operation. Also in the throttle opening degree method, ranges for two operations are defined by a curve C.
As clearly shown in FIG. 1, the fixed air-fuel ratio range above the curve B is wide in the low and middle engine speed zone and is narrow in the high speed zone. It is unfavourable for high speed driving under heavy load conditions, that the fixed rich air-fuel ratio range is narrow in a high speed zone.
On the other hand, using the throttle opening degree detecting method the fixed air-fuel ratio range at low speed zone is narrower than using the vacuum detecting method, but the exhaust gas purification effect is still not sufficient. Further, the fixed air-fuel ratio range at middle and high speed zone is broad, resulting in increases of noxious gas emission.